Monopolar constrained acetabular component

ABSTRACT

The present invention provides monopolar acetabular liners ( 32, 54 ) that can enclose a femoral head ( 14 ) to form a hip replacement prosthesis ( 30, 50 ), wherein the liner ( 32, 54 ) has a rim ( 34 ) that creates an orifice, wherein the orifice has a diameter that is smaller than the diameter of the femoral head ( 14 ), and wherein the liner ( 32, 54 ) encompasses greater than 50 percent of the volume of the head ( 14 ), wherein a constraining ring ( 56 ) can be used in conjunction with the monopolar constrained acetabular liner ( 32, 54 ) to provide additional support for the femoral head ( 14 ) to stay with the acetabular liner ( 32, 54 ) and avoid dislocation. Assemblies and complete prosthetic replacements are also provided. Liners ( 32, 54 ) and constraining rings ( 56 ) having no, one or more recesses ( 36 ) also are provided.

This application is a continuation of U.S. application Ser. No.11/626,608 filed Jan. 24, 2007, which is a divisional of U.S.application Ser. No. 10/144,762 filed May 15, 2002, now U.S. Pat. No.7,169,186, which is a continuation of PCT/US01/22638 filed Jul. 19,2001, which claims benefit of U.S. Provisional No. 60/289,528 filed May9, 2001, U.S. Provisional No. 60/234,345 filed Sep. 22, 2000, and U.S.Provisional No. 60/222,049 filed Jul. 31, 2000. The entire contents ofthe above-identified applications are hereby incorporated by reference.

The present invention relates to improved acetabular components, such asliners, assemblies and complete replacements, for use in the body. Theimproved acetabular components can be used, for example, in hiparthroplasties in both a primary and revision setting. The presentinvention provides a decreased risk of dislocation while facilitatinggreater range of motion (“ROM”).

DESCRIPTION OF THE FIELD

Dislocation is a major source of morbidity at re-operation after totalhip arthroplasty. Dislocation rates have been quoted at 1-10% forprimary total hip arthroplasties. Many studies have shown increasedrates of dislocation after revision total hip arthroplasty when comparedwith primary total hip arthroplasty. Currently, the chance of“successful surgical management of a recurrent dislocation” is 70% whena cause for the dislocation can be identified. Dislocation results insignificant patient anxiety. In addition, the costs associated withsurgical management of dislocation are significant.

A variety of approaches have been used to address dislocation, includingchanging the angle of the femoral component, changing the angle of theacetabular component, using extended lip liners, using off-set liners,advancing the greater trochanter, and using constrained acetabularcomponents. Another approach is to use femoral heads of larger diameter.

Some of these approaches have distinct disadvantages, however. Alteringthe orientation of the components can increase stability in onedirection (for example, posteriorly) but at the same time decreasestability in the opposite direction (for example, anteriorly). Use ofextended lip liners and currently available constrained acetabularcomponents which utilize additional polyethylene to surround the femoralhead increases joint stability but this additional material also can,depending on the design, constrict the range of motion of the joint andcan lead to subsequent problems related to impingement. When the hipjoint is articulated (for example, flexed, extended, rotated, etc.) toits limit, impingement between the femoral neck and acetabular liner mayoccur.

Component impingement is undesirable for several reasons. The femoralneck impinging against the polyethylene liner can produce damage to thepolyethylene as well as increasing the stresses at the component/boneinterface. Additionally, the point of impingement acts as the fulcrumthrough which the force produced by the leg could lever the femoral headout of the liner and hence lead to dislocation.

Many current methods of reducing the risk of dislocation, includingrecurrent dislocation, employ the use of so called “constrained liners,”which are complex bipolar or tripolar articulations. In a study oftwenty one constrained acetabular components to treat preoperative orintraoperative instability, six patients had recurrent dislocations. Itwas postulated that this increase was caused by the impingement of thefemoral stem on the rim of the insert due to decreased range of motionwithin the design. Orthopaedic Knowledge Update, Chapter 38, page 474(James H. Beatty, M.D., editor, AAOS).

Constrained systems that are currently available use ultra highmolecular weight polyethylene (UHMWPE) which is not crosslinked. A majorlimitation of the material is it's high wear rate and the risk ofperiprosthetic osteolysis. Also, their stated range of motion in flexionis between 72° and 90°. Such devices are available from Zimmer, Inc.,Johnson & Johnson/Depuy Inc., and Howmedica-Osteonics, Inc.

The current products have additional limitations. For example, theOsteonics system utilizes only 22, 26 and 28 mm heads. The minimumdiameter of the acetabular shell is 50 mm. This design is “bipolar.” Abipolar system is made up of a acetabular shell with a polyethyleneliner, and inside of that liner is a metal head. Inside of that metalhead is another polyethylene liner, which in turn captures a 22, 26, or28 mm head that is attached to the femoral stem. Thus, the Osteonicssystem has multiple polyethylene on metal articulations. The multiplearticulations generate polyethylene wear debris at a much faster ratethan designs with a single metal on polyethylene articulation.Additionally, the range of motion only is approximately 90 degrees.

A constrained system developed by Zimmer, Inc. uses a metal shell with atwo-piece polyethylene liner. One piece is inserted into the acetabularmetal shell followed by placement of the other portion around the neckon the femoral component before the femoral head has been put on theMorse taper. The head is then reduced into the acetabular component. Thepolyethylene portion with a metal band that is around the femoral neckis then brought into contact with the acetabular polyethylene componentand the metal band then secured to “capture” the head within theassembled polyethylene liner. Again, this design uses ultra highmolecular weight polyethylene and can generate significant debris aswell as a limited range of motion. The limited range of motion resultsin impingement in flexion, which generates a rotational torque at thebone shell interface, potentially inhibiting bony in-growth into theacetabular shell.

In the late 1960's to early 1970's, others attempted to implant acemented conventional UHMWPE liner with greater than 50% head coverage.Lagrange and Letournel, Int Surg. 60(1):21-4 (1975); Lanzetta, ArchOrtop. 83(1):49-53 (1970). These designs had a 35 mm inner diameter andwas available in only two (47 and 58 mm) outer diameters. Theseapproaches, however, did not yield a replacement with acceptableperformance. Long-term data is not available for the LaGrange/Letourneldesign, likely because it did not become a widely-accepted product. Itshead design (35 mm diameter) in conjunction with conventional UHMWPEavailable at the time would have had a substantial wear rate.

Thus, current and past designs have utilized various approaches,including countersinks, in which the center of rotation of the femoralhead is below the level of the flat surface of the liner. Yet even theuse of a countersink design using large head diameters is less thanoptimal because, among other things, these liners did not have anorifice that was less than the diameter of the femoral head, andtherefore have no mechanical capture, and took no other steps to avoidor minimize dislocation.

It has been shown that larger heads are effective in reducing the rateof recurrent dislocations, even when used with conventionalpolyethylene. Amstutz et al., 12th Annual International Symposium forTechnology in Arthroplasty, ISTA '99, Chicago, Ill. (1999). However, hipsimulator studies have shown that the wear of conventional polyethylenewith the larger heads is excessive. This disadvantage was demonstratedin vivo by the results of surface replacement as well. Amstutz et al.,Clin. Orthop. 213: 92-114 (1986). In addition, simply using a large headdiameter does not produce mechanical capture of the femoral head.

In achieving some of the advantages of the invention, in accordance withone aspect of the invention, there are provided liners with one or morerecesses. The type of recesses according to the present invention,however, are different than that of the unconstrained Modell St.Georg/Mark I & II and ENDO-Modell liner, which was to “preventirritation of the psoas muscle and enable(s) removal of cement,” ratherthan for reducing dislocation or impingement. See Englelbrechht E,Siegel A, Kluber D: The Modell St. Georg/Mark I/Mark II Prosthesis. p.66. In: Finerman G, Dorey F, Grigoris P, McKellop H (eds): Total HipArthroplasty Outcomes. Churchhill Livingstone, N.Y., 1998.

In view of the limitations of the commercialized designs, new approachesare needed that would increase the inherent stability of the head-linerarticulation while at the same time increasing range of motion, whichwould result in a decreased rate of dislocation for both primary andrevision total hip arthroplasties. Such approaches would decreasepatient anxiety, eliminate the need for postoperative dislocationprecautions, and reduce the number of revisions performed fordislocation and recurrent dislocation resulting in a net cost savings tothe healthcare system. Such risks of dislocation are markedly reducedwhile range of motion is increased, particularly when a large-headdesign and/or recesses are employed.

Cut away monopolar constrained acetabular liners (U.S. Ser. Nos.60/222,049 and 60/234,345) for use in total hip replacement to helpreduce the potential of dislocation of the hip while still providingsufficient range of motion for daily activities have been developed. Theinvention disclosed herein also provides a separate constraining ringthat can be used in conjunction with the monopolar constrainedacetabular liner and the cut away monopolar constrained acetabular linerto provide additional support for the femoral head to stay with theacetabular liner and avoid dislocation.

SUMMARY OF THE INVENTION

It is an object of the invention to provide improved prostheses for usein the hip. In accomplishing this and other objects, there is providedin accordance with one aspect of the present invention a monopolaracetabular liner that can enclose a femoral head to form a hipreplacement prostheses, wherein the liner has a rim that creates anorifice, and wherein the orifice has a diameter that is smaller than thediameter of the femoral head. The rim extends beyond the center line ofthe head (such as the center of rotation) when the head is enclosed bythe liner, and thereby the head is constrained from dislocation. Theliner can, in certain embodiments, include one or more recesses orcut-aways to facilitate additional range of motion or be without recessor cut away for use with liners that likewise lack a recess or cut away.The liner, in another embodiment, is designed to accommodate properfitting of a constraining ring that can be used in conjunction with themonopolar constrained acetabular liner and the cut away monopolarconstrained acetabular liner. In accordance with another embodiment,there is provided a constraining ring that can be used in conjunctionwith the monopolar constrained acetabular liner and the cut awaymonopolar constrained acetabular liner. The constraining rings provideadditional support for the femoral head to stay with the acetabularliner and avoid dislocation. Preferably, the liner comprises ultra highmolecular weight polyethylene, that more preferably is all or in partcross-linked. Preferably, the cross-linking is performed viairradiation. The femoral head can have a large diameter. Theconstraining ring can, in certain embodiments, include one or morerecesses or cut-aways to be compatible with the corresponding liner.Alternatively, the constraining ring will be without a recess or cutaway for use with liners that likewise lack a recess or cut away.

In accordance with another aspect of the present invention, there isprovided a monopolar acetabular assembly comprising (A) a monopolaracetabular component that encloses a femoral head, wherein the liner hasa rim that creates an orifice, and wherein the orifice has a diameterthat is smaller than the diameter of the femoral head; and (B) a metalshell. The rim extends beyond the center line of the head (such as thecenter of rotation) when the head is enclosed by the liner, and therebythe head is constrained from dislocation. The liner can, in certainembodiments, include one or more recesses or cut-aways to facilitateadditional range of motion or be without recess or cut away; and aconstraining ring which can be used in conjunction with the monopolarconstrained acetabular liner and the cut away monopolar constrainedacetabular liner to provide additional support for the femoral head tostay with the acetabular liner and avoid dislocation. The constrainingring, which can be made from a number of high strength materials (forexample, cobalt chrome alloy, titanium, stainless steel, etc.), isdesigned to fit around the perimeter of the liner. The constrainingring, in another embodiment, is designed to fit properly in conjunctionwith the monopolar constrained acetabular liner and the cut awaymonopolar constrained acetabular liner. Preferably, the liner comprisesultra high molecular weight polyethylene, that preferably is all or inpart cross-linked. Preferably, the cross-linking is performed viairradiation. Preferably, the femoral head has a large diameter. Theconstraining ring can, in certain embodiments, include one or morerecesses or cut-aways to be compatible with the corresponding liner.Alternatively, the constraining ring will be without a recess or cutaway for use with liners that likewise lack a recess or cut away. Inaccordance with the invention, the constraining ring is preferably madeof high strength materials such as cobalt chrome alloy, titanium, orstainless steel.

In accordance with still another aspect of the present invention, thereis provided a hip joint replacement assembly comprising (A) a monopolaracetabular liner that encloses a femoral head, wherein the liner has arim that creates an orifice, and wherein the orifice has a diameter thatis smaller than the diameter of the femoral head; (B) a metal shell; and(C) a femoral head. The rim extends beyond the center line of the head(such as the center of rotation) when the head is enclosed by the liner,and thereby the head is constrained from dislocation. The liner can, incertain embodiments, include one or more recesses or cut-aways tofacilitate additional range of motion or be without recess or cut away;and a constraining ring which can be used in conjunction with themonopolar constrained acetabular liner and the cut away monopolarconstrained acetabular liner to provide additional support for thefemoral head to stay with the acetabular liner and avoid dislocation.The constraining ring, in another embodiment, is designed to fitproperly in conjunction with the monopolar constrained acetabular linerand the cut away monopolar constrained acetabular liner. Preferably, theliner comprises ultra high molecular weight polyethylene, thatpreferably is all or in part cross-linked. Preferably, the cross-linkingis performed via irradiation. The metal shell may be made of titanium,stainless steel, or a cobalt chrome alloy. Preferably, the femoral headhas a large diameter. The constraining ring can, in certain embodiments,include one or more recesses or cut-aways to be compatible with thecorresponding liner. Alternatively, the constraining ring will bewithout a recess or cut away for use with liners that likewise lack arecess or cut away.

In accordance with the invention, metal shells preferably are made oftitanium, cobalt chrome alloys, or stainless steel. The femoral stemspreferably also may be made of a cobalt chrome alloys, stainless steel,or titanium. Preferably, the femoral heads are made of a cobalt chromealloys, stainless steel or ceramic. Where the femoral head and stem areone piece, preferably they are made of a cobalt chrome alloy orstainless steel.

In accordance with still a further aspect of the invention, there areprovided methods of replacing a hips in patients in need thereof,comprising the step of surgically implanting in a given patient amonopolar acetabular liner that can enclose a femoral head to form a hipreplacement prostheses, wherein the liner has a rim that creates anorifice, and wherein the orifice has a diameter that is smaller than thediameter of the femoral head, and thereby the head is constrained fromdislocation. The liner can, in certain embodiments, include one or morerecesses or cut-aways to facilitate additional range of motion or bewithout recess or cut away; and a constraining ring which can be used inconjunction with the monopolar constrained acetabular liner and the cutaway monopolar constrained acetabular liner to provide additionalsupport for the femoral head to stay with the acetabular liner and avoiddislocation. The constraining ring, in another embodiment, is designedto fit properly in conjunction with the monopolar constrained acetabularliner and the cut away monopolar constrained acetabular liner.Implantation of assemblies and total replacements, such as shells,femoral heads and femoral stems, also are provided.

In contrast to current bipolar and tripolar constrained components themonopolar constrained design has: (i) increased range of motion, (ii)decreased wear of the articulation, (iii) simpler intraoperativeassembly, (iv) decreased risk of impingement of the femoral neck uponthe liner rim, (v) decreased stress transfer to the acetabularcomponent-cement interface, (vi) decreased stress transfer to thecement-bone interface, (vii) decreased stress transfer to the metalshell-bone interface, (viii) eliminates the thin polyethylenearticulating surfaces associated with multi-polar designs, (ix)eliminates multipolar locking rings, which have had reportedmalfunctions and failure, (x) allows for the use of large headdiameters, (xi) has smaller possible acetabular shell outer diameters,(xii) utilizes thicker polyethylene bearing surfaces, (xiii) and furtherfacilitate range of motion in preferred directions.

In contrast to the Legrange/Letournel design, the monopolar constraineddesign according to the invention has: (i) a different and superiorbearing surface, (ii) larger head diameters, (iii) decreased wear, (iv)increased ROM, (v) the capacity to be use in cemented or bony in-growthacetabular replacements, (vi) thicker polyethylene bearing surfaces,(vii) modularity for use with uncemented acetabular shells, (viii) canbe easily exchanged at the time of primary or revision surgery, and (ix)and can be designed to facilitate movement in preferred directions.

Although large head diameters with standard UHMWPE bearing surfaces havebeen used for the treatment of dislocation and recurrent dislocation byAmstutz et al. In contrast to the designs employed by Amstutz, themonopolar constrained has: (i) has a mechanical capture of the femoralhead, (ii) reduced wear, (iii) decreased periprosthetic osteolysis, (iv)modularity for easy conversion between different amounts of constraint,and (v) can be designed to facilitate movement in preferred directions.

In embodiments employing the recesses, the liner can have one or morerecess. A recess, often referred to herein as a cut away, can bepositioned to further facilitate movement in a desired direction.

Embodiments employing a constraining ring can be used in conjunctionwith the monopolar constrained acetabular liner and the cut awaymonopolar constrained acetabular liner to provide addition restraintagainst dislocation of the hip joint while at same time not impeding therange of motion of the hip joint. The constraining ring, in anotherembodiment, is designed to fit properly in conjunction with themonopolar constrained acetabular liner and the cut away monopolarconstrained acetabular liner.

The constraining ring can be secured to the acetabular liner by means ofa number of locking mechanisms that are similar to those that arecurrently used to secure acetabular liners into their mating metalacetabular shell. Embodiments of this invention have used a lockingmechanism which relies on the spatial interference between thepolyethylene acetabular liner and the metal acetabular shell. Thisinvention is not limited by the use of the locking mechanism used insuch embodiments and the use of other locking mechanisms are possible.

In another embodiment of this invention have used a locking mechanismwhich relies on the spatial interference between the polyethyleneacetabular liner and the metal constraining ring. As the constrainingring is being seated onto the acetabular liner the polyethylene mustdeform as it moves past a ridge on the constraining ring. Once the ringis fully seated then the deformed polyethylene relaxes behind the ridgehence securing the constraining ring to the liner. This invention is notlimited by the use of the locking mechanism used in such embodiments andthe use of other locking mechanisms are possible. Yet in another theconstraining ring can also be fastened to the acetabular liner by directscrew fixation where screws are passed directly through the constrainingring into the polyethylene liner.

In another embodiment of this invention the constraining ring can besecured to the liner by means of a retaining ring which is designed inan unstressed state to sit in grooves in both the constraining ring andacetabular liner. Upon setting the constraining ring onto the liner theretaining will deform until the constraining ring is fully seated ontothe liner at which point the retaining ring relaxes into the designedgrove in the liner.

The constraining ring can, in certain embodiments, include one or morerecesses or cut aways to be compatible with the corresponding liner.Alternatively, the constraining ring will be without a recess or cutaway for use with liners that likewise lack a recess or cut away.

These and other aspects of the present invention will become apparent tothe skilled person in view of the description set forth below.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a monopolar liner in functional relationwith a femoral head and neck.

FIG. 2 is a perspective view of a monopolar liner containing 2 recessesto facilitate greater range of motion.

FIG. 3 depicts the components of the cut away monopolar constrainedacetabular system with a constraining ring: metal acetabular shell (52)which is secured to the acetabulum, polyethylene cut away monopolarconstrained acetabular liner (54), and constraining ring (56). Note themetal acetabular shell is not essential for this invention as theacetabular liner can be directed secured into the acetabulum with bonecement.

FIG. 4 depicts constraining rings (56) in different perspectives.

DETAILED DESCRIPTION OF ASPECTS OF THE INVENTION

The present invention decreases the risk of dislocation of total hiparthroplasty in both the primary and revision setting. See U.S. Ser. No.60/222,049, filed Jul. 31, 2000, and U.S. Ser. No. 60/234,345, filedSep. 22, 2000, the entirety of which are hereby incorporated byreference. In conjunction with large diameter femoral heads, such asdisclosed in PCT/US99/16070 (the entirety of which is herebyincorporated by reference), the present invention can achieve a range ofmotion that is greater than is currently available so called“constrained” systems using 22, 26, 28 and 32 mm heads. A decrease inthe rate of dislocation will have a positive effect on patientsatisfaction as well as hospital re-admission for treatment ofdislocation. The increased range of motion in this invention will affordpatients a more normal life style than the currently availableconstrained systems. The present invention can be surgically implantedin a patient in the same or similar manner as currently employedimplants. Thus, the present invention results in an improved quality oflife as well as improved patient satisfaction.

The present invention advantageously employs UHMWPE liners, preferablyusing UHMWPE that is cross-linked, including highly cross-linked UHMWPE.UHMWPE can be cross-linked by a variety of approaches, including thoseemploying cross-linking chemicals and/or irradiation. Preferredapproaches for cross-linking employ irradiation, and are taught inPCT/US97/02220, the entirety of which is hereby incorporated byreference.

According to the invention, there is preferably a two part acetabularassembly, namely a metal shell for bony in-growth with UHMWPE liner,preferably where the bearing surface of the liner that comes intocontact with the femoral head is comprised of crosslinked UHMWPE.

The liner of the present invention preferably is “monopolar” system. Themonopolar constrained acetabular preferably has a one piece design andhas a single metal to polyethylene articular surface, and thuspreferably is not bipolar or tripolar in design. The monopolar designallows for the use of thicker polyethylene in the acetabular liner.Compared to the bipolar and tripolar constrained/captured designs, themonopolar design decreases the surface area of contact between metal andpolyethylene because there is only one metal-polyethylene articulation,rather than two or more found in other designs. Moreover, the use ofcrosslinked UHMWPE will decrease the amount of debris particlesgenerated to articulation. Moreover, in the modular form of thisacetabular design, since the polyethylene component is not cemented intoplace, some revision surgeries are more feasible. Moreover, with themodular design different types of heads and liners can be used with theshell, and can be selected by the surgeon during surgery.

According to one aspect of the invention, more than 50% of the volume ofthe femoral head is enclosed within the polyethylene liner. The diameterof the opening/orifice of the polyethylene liner is less than thediameter of the femoral head that is inserted, as shown in FIG. 1.

FIG. 1 depicts at (10) a liner (12) in functional relation with afemoral head (14) and neck (20). The rim surface (16) of the liner (12)extends beyond the center line (18), representing the center of rotationof both the femoral head and the acetabular liner. Preferably the linerrim surface is flat in this embodiment. The inner portions of the rimcircumscribe the orifice. If desired, a metal shell, not pictured, wouldbe positioned between the liner and the bone at the mount site, andpreferably is configured so as to promote in-growth of the bone.

FIG. 2 depicts at (30) a type of recessed or cut away liner (32). In thepictured embodiment, the rim surface (34) is interrupted by two cutaways (36) which permit greater range of motion while preserving aconstraint to hold a head in place by a snap fit. Region (38) shows thatthe rim surface (34) of the liner (32) extends beyond the center line(not specifically depicted) in such a manner that the opening/orifice ofthe liner (32) is less than the diameter of the femoral head that isinserted therein. Thus, the inner portions of the rim circumscribe theorifice in such a manner that the head is held within the liner. Ifdesired, a metal shell, not pictured, would be positioned between theliner and the bone at the mount site, and preferably is configured so asto promote in-growth of the bone.

The embodiments that employ one or more recesses can have the recessespositioned to further facilitate movement in a desired direction. Forexample, a liner might have a single recess to provide increased rangeof motion in flexion. Another arrangement can be a liner with tworecesses with the second recess to provide increased range of motion inextension and/or extension plus external rotation. Moreover, theincreased range of motion provided by the cut-away design also allowsfor greater tolerance for the variations in the orientation of theacetabular component that can occur during insertion.

As stated above, recesses can be positioned to further facilitatemovement in a desired direction, such as in the common directions ofimpingement. Recesses can be placed in several different positions, butpreferentially would be placed in the position to produce the range ofmaximum motion in a desired direction. For example, in a left hip, whenlooking at the acetabular component in position and considering it to bethe face of a clock, the recess would be preferentially placed at about1-2 o'clock. Under similar circumstances one would place a recess for aright hip at about 10-11 o'clock. Recesses in these positions assistwith flexion alone and flexion plus internal rotation. For embodimentswith two or more recesses, at least one recess also would be placed inthe position to maximize extension alone and extension plus externalrotation, and thus these recesses would be placed at about 4-5 o'clockin the left hip and about 7-8 o'clock in the right hip.

The recesses preferably are of sufficient size to accommodate thefemoral head and stem (including the neck), while still constraining thehead within the liner. The size of the recess in terms of width anddepth depend upon the size of the stem neck and the range of motion ofmotion sought, which are readily determinable by the skilled personbased upon a patient's size, age and needs. If the liner is comprised ofa cross-linked polymer, like irradiated ultra high molecular weightpolyethylene, then the recesses preferably are machined into the liner.The liner, in another embodiment, is modified to accommodate aconstraining ring that can be used in conjunction with the monopolarconstrained acetabular liner and the cut away monopolar constrainedacetabular liner to provide additional support for the femoral head tostay with the acetabular liner and avoid dislocation.

FIG. 3 depicts at (50) a type of constraining ring (56). In the picturedembodiment, showing components of the cut away monopolar constrainedacetabular system with a constraining ring: optional metal acetabularshell which is secured to the acetabulum (52), polyethylene cut awaymonopolar constrained acetabular liner (54), and the constraining ring(56). It is notable that the metal acetabular shell is not essential forthis invention as the acetabular liner can be directed secured into theacetabulum with bone cement. If desired, the metal shell (52), would bepositioned between the liner and the bone at the mount site, andpreferably is configured so as to promote in-growth of the bone.

FIG. 4 depicts at (70) a constraining ring (56) shown in differentperspectives.

The opening in the monopolar constrained acetabular liner and the cutaway monopolar constrained acetabular liner through which the femoralhead passes upon reducing the femoral head into the liner is smallerthan the femoral head, which allows a snap-fit. Therefore, theconstrained liner, which is typically made of polyethylene, must deformslightly to allow the femoral head to be embraced by the liner. Theconstraining ring, which can be made from a number of high strengthmaterials (for example, cobalt chrome alloy, titanium, stainless steel,etc.), is designed to fit around the perimeter of the monopolarconstraining acetabular liner and is intended to serve as a structuralsupport of the liner to further ensure that the femoral head will staywithin the acetabular liner.

The constraining ring is circular in shape and has “cutouts” similar tothat of the cut away monopolar constrained acetabular liner (FIGS. 3 &4). This prevents the constraining ring from interfering with the rangeof motion of the hip.

The constraining ring is preferably designed to be a single piece whichcan be set onto the acetabular liner during the operative procedureafter the femoral head has been snap-fitted into the acetabular liner.This allows for the femoral head to stay snapped within the constrainedliner.

The constraining ring can, in certain embodiments, include one or morerecesses or cut-aways to be compatible with the corresponding liner.Alternatively, the constraining ring will be without a recess or cutaway for use with liners that likewise lack a recess or cut away.

Gross sliding or micro motion between separate metallic surfaces of thetotal hip components can generate wear debris which upon release intothe joint space can elicit a biological response that can lead to thedevelopment of osteolysis and hence increasing the of potential ofcomponent loosening. The design of the constraining ring and theacetabular liner are such that to prevent contact between metallicsurfaces. The constraining ring is designed to fit properly inconjunction with the monopolar constrained acetabular liner and the cutaway monopolar constrained acetabular liner.

The components are designed such that the femoral neck will impingeagainst the polyethylene liner and not the metallic constraining ring inall directions of motion. Also, the constraining ring fastens directlyto the polyethylene acetabular liner and has no direct contact with themetal shell into which the liner is set. Another advantage to thisfeature is that the use of the constraining ring is not dependent on theuse of a metal shell, hence allowing this system to be used with anacetabular liner which is cemented directly into the acetabulum.

The constraining ring (see FIGS. 3 & 4) can be secured to the acetabularliner by means of a number of locking mechanisms which are currentlyused to secure acetabular liners into their mating metal acetabularshell. Embodiments of this invention have used a locking mechanism whichrelies on the spatial interference between the polyethylene acetabularliner and the metal acetabular shell. This invention is not limited bythe use of the locking mechanism used in such embodiments and the use ofother locking mechanisms are possible.

In another embodiment of this invention have used a locking mechanismwhich relies on the spatial interference between the polyethyleneacetabular liner and the metal constraining ring. As the constrainingring is being seated onto the acetabular liner the polyethylene mustdeform as it moves past a ridge on the constraining ring. Once the ringis fully seated then the deformed polyethylene relaxes behind the ridgehence securing the constraining ring to the liner. This invention is notlimited by the use of the locking mechanism used in such embodiments andthe use of other locking mechanisms are possible and contemplated by theinvention.

Yet in another the constraining ring can also be fastened to theacetabular liner by direct screw fixation where screws are passeddirectly through the constraining ring into the polyethylene liner.

In another embodiment of this invention the constraining ring can besecured to the liner by means of a retaining ring which is designed inan unstressed state to sit in grooves in both the constraining ring andacetabular liner. Upon setting the constraining ring onto the liner theretaining will deform until the constraining ring is fully seated ontothe liner at which point the retaining ring relaxes into the designedgrove in the liner.

The constraining ring can, in certain embodiments, include one or morerecesses or cut-aways to be compatible with the corresponding liner.Alternatively, the constraining ring will be without a recess or cutaway for use with liners that likewise lack a recess or cut away.

Description of the Assembly of Cut Away Monopolar Constrained AcetabularLiner with Constraining Ring (see FIG. 3):

Components of the cut away monopolar constrained acetabular systemconsisting of: i) metal acetabular shell which is secured to theacetabulum, ii) polyethylene cut away monopolar constrained acetabularliner, and iii) constraining ring. The metal acetabular shell is notessential for this invention as the acetabular liner can be directedsecured into the acetabulum with bone cement. The cut away monopolarconstrained acetabular liner is set into the metal acetabular. Theconstraining ring is placed over the femoral component and the femoralhead is snap-fitted into the acetabular liner. The constraining ring isthen set onto the cut away monopolar constraining acetabular liner withthe cutouts of the constraining ring aligned with the cutouts of theacetabular liner.

Demonstration of the Position of the Acetabular Component within the Hipand Range of Motion:

The actual position of the acetabular component within the hip will beset during the operative procedure and will depend on the patient'sanatomy and gait. The cut away portion of the acetabular componentallows for sufficient range of motion in flexion. The cut away portionof the acetabular component allows for sufficient internal rotation at90° of flexion. The cut away portion of the acetabular component allowsfor sufficient external rotation at neutral flexion.

In any of the embodiments of the invention, the liners of the inventioncan be used with hemispherical and non-hemispherical acetabular shells.

Also, the invention can be used with any head diameter, including largehead diameters (for example, 35 mm or greater, such as 38 mm, 40 mm, 42mm, 44 mm, and any diameter in between or larger) in order to increasethe range of motion in comparison with currently available constrainedsystems.

The invention is further demonstrated by the following example, which donot limit the invention in any manner.

Example 1

The load required for insertion and dislocation of femoral heads in aconstrained liner according to the invention was quantified by inserting32 and 38 mm heads into simulated liners of electron beam crosslinkedUHMWPE. Two and three dimensional modeling was performed to assess ROMand stability in vitro. The liner had no chamfer.

The ROM, and the loads required for insertion and dislocation usingdifferent opening diameters for 32 mm and 38 mm heads in simulatedmonopolar liners were measured. A 32 mm head with a 31 mm liner openingrequired 60 lbs. for insertion and 55 lbs. for dislocation. When theopening was increased to 31.5 mm the forces for insertion anddislocation decreased to 29 lbs., and 24 lbs., respectively. Testing of35.5 mm, 36.5 mm, and 37.5 mm openings with a 38 mm head was performed.The insertional loads were 157, 130, and 28 lbs. The force necessary toproduce dislocation of the femoral head were 135, 126, and 28 lbs.,respectively.

The ROM for the 38 mm head and a 35.5 mm orifice liner is 110°. Thisincreases to 118° and 131° as the orifice diameter is increased to 36.5and 37.5 mm. The 32 mm head and 31 mm orifice liner has 116° ROM. TheROM increases to 124° when a 31.5 mm orifice is used with the 32 mmhead.

These data demonstrate that the constrained design according to theinvention can minimize the occurrence of dislocation, even with headsand liners that are constructed to permit enhanced range of movement.

It is to be understood that the description, specific examples and data,while indicating exemplary embodiments, are given by way of illustrationand are not intended to limit the present invention. Various changes andmodifications within the present invention will become apparent to theskilled artisan from the discussion, disclosure and data containedherein, and thus are considered part of the invention.

The invention claimed is:
 1. A metallic constraining ring that fastensdirectly to a cut-away monopolar constrained acetabular liner, whereinthe constraining ring has at least one positioned recess that iscompatible with at least one recess on the cut-away monopolarconstrained acetabular liner that comprises cross-linked ultrahighmolecular weight polyethylene, wherein the metallic constraining ringfits around the perimeter of the cut-away monopolar constrainedacetabular liner, and wherein the metallic constraining ring is set ontothe cut-away monopolar constrained acetabular liner during the operativeprocedure after the femoral head is reduced into the cut-away monopolarconstrained acetabular liner, wherein following the operative procedurethe metallic constraining ring is not impinged by a femoral neck,wherein at least one recess can be positioned to increase range ofmotion.
 2. The metallic constraining ring according to claim 1, whereinthe shape of the metallic constraining ring is circular.
 3. The metallicconstraining ring according to claim 1, wherein metallic constrainingring is made of at least one selected from the group consisting ofcobalt chrome alloy, titanium, and stainless steel.
 4. The metallicconstraining ring according to claim 1, wherein the ring is a singlepiece.
 5. The metallic constraining ring according to claim 1, whereinthe metallic constraining ring has at least two recesses that arecompatible with at least two respective recesses on the cut-awaymonopolar constrained acetabular liner, wherein the first recess of themetallic constraining ring increases flexion and the second recess ofthe metallic constraining ring increases extension.
 6. A hip jointreplacement assembly comprising: (a) a monopolar constrained acetabularliner for enclosing a femoral head, wherein the liner has a rim thatcreates an orifice, wherein the orifice has a diameter that is smallerthan the diameter of the femoral head, wherein the femoral head has adiameter of 32 mm or larger, wherein the monopolar constrainedacetabular liner comprises cross-linked ultrahigh molecular weightpolyethylene; (b) a metallic constraining ring that fastens directly tothe monopolar constrained acetabular liner and fits around the perimeterof the monopolar constrained acetabular liner, wherein the metallicconstraining ring is capable of being set onto the acetabular linerduring an operative procedure after the femoral head has been reducedinto the monopolar constrained acetabular; and (c) a femoral headattached to a femoral neck, wherein during movement the femoral neck canimpinge upon the monopolar constrained acetabular liner and not themetallic constraining ring.
 7. The hip joint assembly according to claim6, wherein the metallic constraining ring can be used without a metalshell, thereby allowing the monopolar constrained acetabular liner to becemented directly into the acetabulum.
 8. The hip joint assemblyaccording to claim 6, wherein the metallic constraining ring can besecured to the acetabular liner by at least one locking mechanism. 9.The hip joint assembly according to claim 6, wherein the shape of themetallic constraining ring is circular.
 10. The hip joint assemblyaccording to claim 6, wherein the metallic constraining ring is made ofat least one selected from the group consisting of cobalt chrome alloy,titanium, and stainless steel.
 11. The hip joint assembly according toclaim 6, wherein the metallic constraining ring is a single piece. 12.The hip joint assembly according to claim 6, wherein the monopolarconstrained acetabular liner is a cut away monopolar constrainedacetabular liner.
 13. The hip joint assembly according to claim 6,wherein the metallic constraining ring has no recess.
 14. The hip jointassembly according to claim 6, wherein the metallic constraining ringhas at least one recess.
 15. The hip joint assembly according to claim6, wherein the metallic constraining ring has at least two recesses. 16.The hip joint assembly according to claim 6, wherein the liner has norecess.
 17. The hip joint assembly according to claim 6, wherein theliner is a cut-away liner that has at least one recess.
 18. The hipjoint assembly according to claim 6, wherein the liner is a cut-awayliner that has at least two recesses.
 19. A hip joint replacementassembly comprising: (a) a cut-away monopolar constrained acetabularliner for enclosing a femoral head, wherein the cut-away monopolarconstrained acetabular liner comprises cross-linked ultrahigh molecularweight polyethylene, and has (i) a rim that creates an orifice that hasa diameter that is smaller than the diameter of the femoral head, and(ii) at least one recess; (b) a metallic constraining ring that fastensdirectly to the monopolar constrained acetabular liner and fits aroundthe perimeter of the cut-away monopolar constrained acetabular liner andhas a recess that is compatible with the recess on the cut-awaymonopolar constrained acetabular liner; and (c) a femoral head attachedto a femoral neck, wherein during movement the femoral neck can impingeupon the monopolar constrained acetabular liner and not the metallicconstraining ring.
 20. The hip joint replacement assembly of claim 19,wherein the cut-away monopolar constrained acetabular liner has firstand second recesses and the metallic constraining ring has first andsecond recesses that are respectively compatible with the first andsecond recesses on the cut-away monopolar constrained acetabular liner.21. The hip joint replacement assembly of claim 20, wherein the firstrecesses on the cut-away monopolar constrained acetabular liner and themetallic constraining ring assist with flexion and the second recesseson the cut-away monopolar constrained acetabular liner and the metallicconstraining ring maximize extension.
 22. The hip joint replacementassembly of claim 21, wherein the first compatible recesses increaseflexion and the second compatible recesses increase extension.
 23. Thehip joint replacement assembly of claim 19, wherein the femoral head hasa diameter of at least 32 mm.
 24. The hip joint replacement assembly ofclaim 23, wherein the femoral head has a diameter greater than 32 mm.25. The hip joint assembly according to claim 19, wherein the metallicconstraining ring is made of at least one selected from the groupconsisting of cobalt chrome alloy, titanium, and stainless steel.